The present invention relates to methods and apparatus for investigating subsurface earth formations traversed by a borehole and, more particularly, to methods and apparatus for distinguishing between permeable and impermeable zones in the earth formation through the use of pad-mounted electrode systems traversing the borehole that determine the presence of mud cake.
When drilling a well, it is customary to use a drilling mud containing fine, solid particles in suspension. As the hydrostatic pressure of the mud column in the borehole is generally higher than the internal pressure of the earth formation, the fluid portion of the drilling mud has a tendency to penetrate into permeable layers or zones of the earth formation leaving a mud cake formed along the borehole wall. The fluid which penetrates into the permeable layers also drives out, at least partially, the fluid originally contained in the formation.
On the other hand, there is practically no mud cake on the borehole wall opposite impermeable subsurface earth formations since the mud fluid cannot penetrate into the impermeable formation. Knowledge of the presence or absence of mud cake, therefore, permits a differentiation to be made between permeable and impermeable layers in the formation. Since hydrocarbons are generally found in permeable layers in the formation, the presence or absence of a mud cake can provide an indication of the location of hydrocarbons below the surface of the earth.
One example of a well logging tool that has been developed to provide an indication of mud cake is the Microlog (a trademark of Schlumberger Limited) tool. That apparatus includes a pad-mounted electrode system, which is lowered into a borehole and from which one current is emitted into the formation and two voltages are measured at different distances from the center of the electrode structure. The two measurements provide an indication of the resistivity of the formation near the borehole (shallow resistivity) and at a distance further away from the borehole (deep resistivity); that is, measurements of resistivity of two zones in the formation at different distances from the borehole wall. By comparing the resistivity values, an indication of the presence or absence of mud cake in the formation can be obtained. For example, if the deep resistivity measurement is found to be greater than the shallow resistivity measurement, the presence of a mud cake is indicated, which is an indication that the formation is permeable. If, however, the deep resistivity reading is less than the shallow resistivity reading, the absence of a mud cake is indicated, which is an indication that the formation is impermeable. While low resistivity readings of either the deep or shallow resistivity could prove to be ambiguous, experience has shown that such low resistivity readings are generally obtained opposite impermeable shale zones in the earth formation.
More recently, pad-mounted electrode tools have been developed that provide greater accuracy in obtaining the deep and shallow resistivity measurements. This new spherically-focused logging tool is described in detail in U.S. Pat. No. 3,760,260, which issued on Sept. 18, 1973 to Nick A. Schuster and is assigned to the assignee of the present application. In the system disclosed therein, current electrodes associated with alternating current generators emit a main current, I.sub.0, and an auxiliary current, I.sub.1, from a central electrode. One of the current generators is controlled in accordance with the potential difference measured on the outer surface of the pad at a selected location in the borehole so that the auxiliary current forces the main current to penetrate into the formation and so that the auxiliary current flows primarily in the mud cake between the pad and the formation or, in the absence of mud cake, in the borehole and earth formation adjacent the borehole. The necessary voltages and currents are monitored by appropriate electronic circuitry to provide the desired values of deep and shallow resistivity.
It has been determined, however, that despite the fact that more accurate resistivity values can be obtained with the spherically-focused, pad-mounted tool identified above, the response of the tool in the presence of mud cake is not always accurate. While in the presence of mud cake, the shallow resistivity is much less than the deep resistivity measurement, and in the absence of mud cake, the shallow resistivity measurement is approximately seven times the deep resistivity measurement, nevertheless, it is difficult to distinguish between the presence of mud cake on the borehole wall and the presence of shale in the formation.
There have been previous attempts to utilize a pad-mounted, spherically-focused electrode tool to obtain information regarding the mud cake on the borehole wall. Two co-pending applications, Ser. Nos. 324,906 and 324,907, both filed Jan. 19, 1973, and now abandoned, in the names of Suau and Attali et al, respectively, each disclose a method and apparatus for obtaining the information concerning the mud cake thickness. However, while these disclosed methods and apparatus do provide accurate results, they do still suffer from the problem of the difficulty in distinguishing between mud cake and shale.
The difficulties in distinguishing between mud cake and shale when using a spherically-focused electrode tool are to a large extent overcome by the techniques disclosed in co-pending U.S. patent application Ser. No. 490,377 of Vieiro filed July 22, 1974 and assigned to the assignee of the present application. According to the technique disclosed by Vieiro, the main and auxiliary currents emitted from the spherically-focused tool are detected and a first parameter produced related solely to the auxiliary current and a second parameter produced related to both the main and auxiliary currents. The technique described in the above-identified Vieiro application, however, did not prove accurate in all types of formations and, in particular, inaccuracies could occur in measurements opposite permeable formations when the ratio of the deep resistivity to the mud resistivity exceeded 10.